This invention relates to a method for the production of a porous material by the polymerization of a water-in-oil type high internal phase emulsion (hereinafter referred to as xe2x80x9cHIPExe2x80x9d), which is enabled by adding at the step of production a compound capable of reacting with a polymerization initiator and/or a compound capable of reacting with an unsaturated double bond in order to maintain the quality thereof stably for a long time and to excel in the ability.
As a technique for obtaining a porous material formed of continuous open cells having a fine and uniform diameter, which comprises forming an HIPE in the presence of a specific surfactant and then obtaining a polymer of the HIPE has been known. The term xe2x80x9cHIPExe2x80x9d as used herein is generally known as an emulsion of such a quality that the dispersed phase thereof is exceeding 70 vol. % of the whole volume of the emulsion. The official gazette of U.S. Pat. No. 5,334,621, for example, discloses a method for producing a porous material by the HIPE, which method is subjected a polymerizable monomer contained in such an HIPE to cross-linking polymerization.
The HIPE is prepared, for example, from (i) a polymerizable monomer mixture composed of an oil-soluble vinyl monomer and a cross-linking monomer possessing not less than two functional groups in the molecular unit thereof, (ii) a water phase being 90 wt %, preferably 95 wt %, and particularly preferably 97 wt % of the total amount of the emulsion, (iii) a surfactant such as, for example, the sorbitan fatty acid ester or the glycerol monofatty acid ester, and (iv) a polymerization initiator. The porous material can be produced by heating, polymerization and cross-linking the HIPE. The porous material produced by this HIPE method is composed of continuous open cells forming a reticular pattern. The porous material obtained by the HIPE method, therefore, has such characteristic properties as low density, water absorbing property and water retaining property, heat insulating property, and sound insulating property. It can be used as absorbents for sanitary materials such as disposable diapers and sanitary napkins, as sound insulators and heat insulators for absorbing sound and heat, and as bases for impregnation with aromatic materials and detergents. It can further be used as absorbents for oils and organic solvents.
For the purpose of producing the porous material by the HIPE method with satisfactory commercial productivity, it is necessary that the relevant reaction be completed quickly. Thus, the reaction prefers use of a polymerization initiator in a comparatively large amount, but the obtained porous material is found to incur the problem that the properties thereof are changed by age. Specifically, the speed of water absorption, the absorbent capacity, and the strength of material which are required to the porous material as product properties show a discernible sign of change by age after standing at rest under specific conditions.
The fact that the porous material properties change during the course of protracted storage, when considered on the assumption that the porous material is used as the absorbent for a sanitary material, for example, means that the performance of a disposable diaper or a sanitary napkin varies with the elapse of time. This is not agreeable.
Further, when the porous material is quickly obtained by using the polymerization initiator in a comparatively large amount as described above, it possibly suffers persistence of the polymerization initiator or the monomer therein. From the viewpoint of safety, the porous material is preferred to have such residual components diminished.
Heretofore, the method for washing the porous polymer obtained by polymerizing the HIPE has been known. When this method is applied in an effort to diminish the residual polymerization initiator or the residual monomer, however, the amount of the water to be used for the washing reaches several times to some tens of times and, under the worst conditions, some hundreds of times as large as the amount of the porous material and the process of washing becomes proportionately complicated. The disposal of the washings also requires an extra process and eventually boosts the cost of production greatly.
No method has been heretofore known to be capable of producing such a porous material as encountering no change by age, maintaining performance stably, and excelling in safety by a process of satisfactorily high productivity without entailing any complicated step.
The present inventors, after pursuing a study in search of a process for producing a porous material by the HIPE method, have found that a porous material exhibiting an excellent shelf life and possessing a satisfactory water absorbing property is obtained by adding a compound capable of reacting with a polymerization initiator and/or a compound capable of reacting with an unsaturated double bond to the HIPE, the porous polymer, or the porous material at a specific stage of polymerization and that a porous material having a residual polymerization initiator and a residual monomer only at low levels and excelling in safety is obtained without undergoing a complicated process such as the washing step. This invention has been perfected as a result.
According to this invention, by adding a compound capable of decomposing a polymerization initiator and/or a compound capable of reacting with an unsaturated double bond at the step of production of the porous material, it is made possible to exalt immensely the stability of the porous material in storage and diminish the amounts of the residual polymerization initiator and residual monomer in the product as well.